The present invention relates to a guide roller for guiding a magnetic tape in the magnetic tape cassette and a method of manufacturing the same.
Conventionally, a magnetic tape cassette is widely used as a storage medium. The magnetic tape cassette is divided into ones for consumer use and ones for industrial use depending on the purpose. FIG. 20 is an exploded perspective view showing main components of a BETACAM L cassette that is a magnetic tape cassette for commercial use.
As shown in FIG. 20, the BETACAM L cassette (hereinafter referred to as xe2x80x9cmagnetic tape cassettexe2x80x9d) 20 generally comprises an upper cassette half 24 and a lower cassette half 22 constituting a cassette case, a pair of tape reels which is rotatably received in the upper and lower cassette halves 24, 22 and on which a magnetic tape 26 is wound, and a lid 32 for opening and closing the opening formed on the front end side of the upper and lower cassette halves 24 and 22.
On the lower cassette half 22, guide rollers 36 and metallic guide pins 38 are mounted at both sides of the opening on the front end side across which a magnetic tape 26 is to be routed. In addition, pads 44 to be in sliding contact with the back side of the magnetic tape 26 is mounted on the lower cassette half 22. On the upper cassette half 24, there are provided reel springs 28 for urging the respective tape reels 28, via the reel holders 46, and center caps 50 for engaging the reel springs 48 from outside of said cassette half 24. The upper and lower cassette halves 24, 22 are fixed together by a plurality of screws 34.
Among components described above, the guide roller 36 is a cylindrical member formed of a smooth and mechanically strong resin such as POM, and rotatably supported on the lower cassette half 22 by being loosely fitted on the trunnion pin 36A fixed on the lower cassette half 22.
FIG. 21 is an enlarged view of the guide roller 36 and the guide pin 38 on the lower cassette half 22 viewed in the direction shown by the arrow B in FIG. 20. In FIG. 21, there is shown a state where the magnetic tape 26 is routed from the tape reel 28 (See FIG. 20) over the guide roller 36 and the guide pin 38. The pad 44 is in sliding contact with the back surface of the magnetic tape 26 between the tape reel 28 and the guide roller 36 and urges the magnetic tape 26. As a consequent, the magnetic tape 26 comes into contact with the outer periphery of the guide roller 36. When the magnetic tape cassette 20 is loaded on the recording/replaying apparatus, not shown, and replayed or rewound, the magnetic tape 26 is passed on the outer periphery of the guide roller and then routed out from the cassette case and then travels over the magnetic head of the recording/replaying apparatus.
FIG. 22 is a cross sectional view taken along the line Cxe2x80x94C in FIG. 21. The guide roller 36 is cylindrical in shape and the outer periphery thereof has a uniform diameter.
In the guide roller 36 of the magnetic tape cassette 20 as described above, there are cases where the magnetic tape 26 being in contact with the guide roller 36 is displaced from the normal position upwardly along the axis of said guide roller or downwardly along the same though it is not shown, and thus the magnetic tape 26 does not travel stably. As a consequent, when the magnetic tape 26 is traveling over the magnetic head of the recording/replaying apparatus, the position of the magnetic tape 26 may be displaced with respect to the magnetic head, and thus reading or writing of the record cannot be accomplished properly. This is a problem.
On the other hand, when molding a guide roller 36 described above by injection molding, the parting line PL of the mold is normally defined along the end surface of the molded object. However, it has been a very troublesome work to take an elongated guide roller out of the mold. Though it is also possible to form the guide roller 36 by cutting a bar by machining, it requires a long process time. In addition, since a costly machine such as NC lathe is required for such a cutting work, the processing cost increases to a significant level.
In order to solve this problem, the parting line of the injection mold may be defined along the center portion or along the axis thereof. However, defining the parting line in this way generates molding burr that projects all along the periphery thereof, and thus the secondary process such as cutting work must be made by a machine for deburring, which results in increase in time and effort. This is also a problem.
In addition, since the guide roller 36 shown in FIG. 22 guides the magnetic tape 26, it is supported with good rotatability, and thus when the magnetic tape 26 travels at high velocity such as fast forwarding operation or fast rewinding operation, the guide roller 36 rotates at high velocity accordingly. Consequently, at least one of the guide roller 36 and the upper and lower cassette halves 24, 22 is cut down by friction between the upper and lower end surfaces of the guide roller 36 and the inner surfaces of the upper and lower cassette halves 24, 22, and resultant cutting powder may be attached on the magnetic tape 26 and cause a dropout.
It has been shown that when the magnetic tape 26 is running at low velocity for replaying the record, the guide roller also rotates at a low velocity and the upper and lower end surfaces of the guide roller 36 comes into contact with the inner surfaces of the upper and lower cassette halves 24, 22, but cutting powder is not generated during low velocity rotation. This is a problem.
It is an object of the present invention to solve the above-mentioned problem in the conventional technique.
With such problems in view, the first object of the present invention is to provide a magnetic tape cassette in which the magnetic tape can travel stably so that the magnetic head can accomplish reading and writing of the record on the magnetic tape correctly.
With this problem in view, the second object of the present invention is to provide a guide roller that allows the magnetic tape to travel stably so that reading and writing of the record on the magnetic tape can be accomplished correctly without generating molding burr on the circumferential surface along the parting line, and the method of manufacturing the same.
With the problem described above in view, the third embodiment of the present invention is to provide a guide roller for the magnetic tape cassette in which generation of cutting powder at the contact point between the guide roller and the upper and lower cassette halves is minimized, and the magnetic head can carry out reading and writing of the record on the magnetic tape correctly.
In other words, said first object of the present invention is accomplished by a magnetic tape cassette comprising a pair of tape reels with a magnetic tape wound therearound and a guide roller for transferring said magnetic tape rotatably supported within a cassette case formed by combining the upper cassette half and the lower cassette half so that said magnetic tape is guided by said guide roller and travels at a prescribed position, wherein said guide roller is generally barrel shaped, and a coefficient of curvature T expressed by the expression [1] which represents the extent of curvature of the outer circumferential surface thereof is between 0.001 and 0.01 inclusive.
T=xcex94/(L"PHgr"max)xe2x80x83xe2x80x83[1]
Wherein xcex94 represents a difference between the maximum diameter and the minimum diameter of the outer circumferential surface of the guide roller (mm), L represents the axial length of the guide roller, and "PHgr"max is a maximum diameter (mm) of the circumferential surface of the guide roller (mm).
The present Inventor has found that the problems described above may be solved by employing a barrel shaped guide roller and defining the extent of curvature of the outer circumferential surface within a prescribed range.
In the magnetic tape cassette having a structure as described above, the circumferential velocity of the portion of the outer circumferential surface of the guide roller having a maximum diameter is faster than the circumferential velocity of other portion. The magnetic tape traveling on the outer circumferential surface of the guide roller is guided to the position of said guide roller of which the circumferential velocity is faster. In addition, in the guide roller of this type, since the coefficient of curvature T representative of the extent of curvature of the outer circumferential surface in the range between 0.001 and 0.01 inclusive, the magnetic tape travels stably without being displaced from the correct position during its travel on the outer circumferential surface, and thus error in reading and writing is prevented from occurring when the magnetic tape is traveling over the magnetic head.
Though a guide roller having a curved outer circumferential surface such that the circumferential diameter is maximum at the axially central portion and gradually decreases from the axially central portion toward both ends may be employed, it is not limited thereto. For example, it is also possible to make the circumferential diameter at a prescribed position of the guide roller maximum so that the magnetic tape may be guided at a desired position.
The second object of the present invention is accomplished by the guide roller of the magnetic tape cassette stated below in (1) to (4), and the method of manufacturing the same.
(1) A guide roller of the magnetic tape cassette comprising a tubular primary molding as an outer jacket, and injection resin as content to be bulged within the core and cavity plates formed in a barrel shaped cavity having a maximum diameter at the parting line.
(2) A method of manufacturing a guide roller for the magnetic tape cassette comprising steps of: injecting resin between a sleeve being slidable with respect to an insert pin and a first cavity plate having a cylindrical cavity to mold a tubular primary molding; leaving said primary molding by withdrawing said first cavity plate; moving a mold comprising a core and a cavity plates having two halves of barrel shaped cavities having its maximum diameter at its parting line and a gate defined within said primary molding onto the outer periphery of said primary molding; and withdrawing said sleeve, injecting resin between the inner side of said primary molding and said insert pin through said gate, and bulging said primary molding into said barrel shaped cavity by the injecting pressure to integrate said primary molding with resin injected within said primary molding.
(3) A method of manufacturing a guide roller of a magnetic tape cassette comprising steps of: molding a tubular primary molding by arranging a slidable double sleeves within a barrel shaped cavity half of the core plate, providing a pin for opening/closing the gate facing toward the barrel shaped cavity half of the cavity plate, then opening a gate by the action of said gate opening/closing pin with said double sleeve projected into said cavity when closing said mold formed of both plates and injecting resin between said double sleeves to form a tubular primary molding; leaving said primary molding body within a cavity by closing said gate and withdrawing said double sleeves; opening said gate again, injecting resin within said primary molding through said gate and bulging said primary molding into the shape of said barrel shaped cavity by the injection pressure to integrate said primary molding with resin injected within said primary molding.
(4) A method of manufacturing a guide roller of the magnetic tape cassette wherein a mold comprises a core plate, a cavity plate, and a runner plate being able to be detached from said cavity plate and having a gate at the tip thereof, further comprising steps of: molding a tubular primary molding by injecting resin between said double sleeves through a gate of the runner plate with the slidable double sleeves projected within said cavity when closing the mold of each plate, leaving the primary molding within said cavity, injecting resin again within the primary molding through said gate of said runner plate and bulging said primary molding into the shape of said cavity by the injection pressure to integrate said primary molding with resin injected within said primary molding, and cutting the gate by detaching said runner plate.
In a guide roller of the magnetic tape cassette as set forth in (1) above, since the already molded primary molding comes into contact with the parting line of the cavity at the time of injection molding, there is no possibility that a molding burr is generated along the outer circumferential surface of the guide roller. The guide roller as set forth in (1) above is specifically effective for the guide roller having a barrel shaped outer periphery with the maximum diameter at a prescribed location in the axial direction. In other words, it is effective when the guide roller cannot be forced out utilizing resiliency of resin since the bulged large diameter portion of the barrel shaped guide roller does not become an undercut.
The outer jacket of the guide roller formed of the primary molding is preferably situated behind the end surface of the guide roller that is a molding. It is for preventing the end of the primary molding from interfering with rotation of the guide roller in the state being fitted on the pin with the end surface being seated on the bottom surface of the cassette half.
Since resin resists flowing when the thickness of the primary molding of the guide roller is less then 0.1 mm, and resists bulging into the barrel shape when the thickness exceeds 1.0 mm, it is preferable to set the thickness of the primary molding in the range between 0.1 mm and 1.0 mm inclusive.
According to a method of manufacturing as set forth in (2) above, the guide roller having a first molding as an outer jacket is molded by molding a tubular primary molding outside the sleeve, then withdrawing the sleeve, and injecting resin within the primary molding.
The method of molding as set forth in (3) above is so called hot runner method, wherein a first molding and the body portion are formed in two stages in the identical molding by the movement of the sleeve and the pin for opening/closing the gate at the time of mold closing. Therefore, replacement of the cavity mold is not necessary.
The method of molding as set forth in (4) above is so called cold runner method, wherein the primary molding and the body portion are formed in two stages in the identical mold by the combination of the movement of the sleeve and the presence of absence of the supply of resin from the runner plate at the time of mold closing. Therefore replacement of the cavity mold is not necessary.
The third embodiment of the present invention is accomplished by the structure shown below.
(5) A guide roller for a magnetic tape rotatably supported in the vicinity of the opening formed on the magnetic tape cassette for exposing the magnetic tape for guiding the forwarding and rewinding operation of the magnetic tape, wherein said guide roller has a shape that can take air around the outer periphery when said magnetic tape is traveling at high velocity.
(6) A guide roller for a magnetic tape cassette as set forth in (5) above, wherein the axially central potion of the circumferential surface of said guide roller is a flat surface.
(7) A guide roller for a magnetic tape as set forth in (5) above, wherein the radius of curvature of the circumferential surface thereof is larger at the axially central portion than the axially end portions.
(8) A guide roller for a magnetic tape as set forth in (7) above, wherein the distance between the circumferential surface of said axially central portion and the extended circular arc surface at said axially ends portion is not less than 0.05 mm.
(9) A guide roller for a magnetic tape cassette as set forth in any one of (5) to (8), wherein said guide roller is provided with a recess at the axially central portion of the circumferential surface thereof.
(10) A guide roller for a magnetic tape cassette as set forth in (9) above, a plurality of said recesses are formed circumferentially at adequate intervals with respect to each other.
(11) A guide roller for a magnetic tape cassette as set forth in (9) or (10) above wherein the depth of said recess at the axially ends from the extended circular arc surface is between 0.05 mm and 0.3 mm inclusive.
In the guide roller for a magnetic tape cassette having a structure described above, since air is taken in around the circumferential surface of the guide roller when the magnetic tape is traveling at high velocity, adhesion between the magnetic tape and the guide roller is lowered, thereby reducing the followability of the guide roller to the magnetic tape traveling at high velocity. Therefore, the guide roller slips with respect to the magnetic tape traveling at high velocity all the time or intermittently. Therefore, even when the magnetic tape travels at high velocity, cutting powder can be prevented from being generated from at least one of the guide roller and the upper and lower cassette halves.
In order to construct in such a manner that air is taken-in around the circumferential surface of the guide roller when the magnetic tape is traveling at high velocity, for example, the axially central portion of the circumferential surface of the guide roller may be formed in a plane surface as in (6), the radius of curvature of the circumferential surface of the guide roller may be defined to be lager at the axially central portion than the axially ends portion as in (7), or a recess may be provided at the axially central portion of the circumferential surface of the guide roller as in (8). Though the recesses stated in (8) are preferably provided all around the circumferential region, it is also possible to provide them along a part of the circumference at adequate intervals.